1. Field of the Invention
The present invention relates to an apparatus for fixing electronic parts to a printed circuit board or substrate, and more particularly to an apparatus suitable for attaching to a high density printed circuit board a variety of electronic parts such as resistors, capacitors, and so-called flat packaged elements from which four-way electrode terminals are extracted in a planar manner.
2. Description of the Prior Art
Recently, a higher density attachment of electronic parts to a printed circuit board has been developed. Since a soldering or brazing operation for attaching electronic parts such as semiconductor chips to a printed circuit board is attained in a final step of the working line, a performance of the electronic parts largely depends upon the quality of the soldering work. Thus, the soldering technique becomes the most important and significant technique in various works in the manufacturing line.
At present, a vapor reflow type soldering apparatus has been proposed which employs a vapor having a large specific weight relative to that of air as thermal medium and heats material to be processed by utilizing its condensing latent heat, in view of needs to enhance a uniformity of temperature distribution within a soldering work furnace and to avoid an undesirable excessive heating of the electronic parts.
In this apparatus, the electronic parts are mounted on a soldering pattern on the printed circuit board, and the printed circuit board is caused to pass through saturated vapor of thermal medium having a large specific weight relative to that of air as described above, by delivering means such as a conveyor, so that the solder on the board is heated and melted. As a result, the electronic parts are soldered on the printed circuit board. Thus, the apparatus has a vapor tank which is a so called vapor reflow reservoir. More specifically, the thermal medium used in such a vapor tank generally comprises a fluorine system inactive organic agent. Its saturated vapor may have a large specific weight (relative to that of the air) of about 20 times, with its molecular weight being at about 820 g/mol under an intended temperature/pressure condition.
Such an apparatus suffers from the following disadvantages.
(1) If the solder for the components to be soldered would be molten within the vapor tank, a part of the solder and a part of flux of the soldering solvent coated on the components to be soldered would fall into a bottom portion of the vapor tank together with the condensed and liquefied thermal medium to be mixed with the stagnant thermal medium. The flux mixed with the thermal medium would stick to a surface of a heater dipped in the thermal medium to reduce its heat transfer performance and to corrode the heater. Thus, there is a high maintenance cost.
(2) The flux mixed with the thermal medium would prevent the boiling action of the thermal medium to delay a responsibility to a change in thermal load. It would be temporarily difficult to maintain constant a level of the vapor surface of the saturated vapor. In order to avoid this phenomenon, it would be necessary to increase the vapor amount, which would need an increased electric power of the heater and an increased amount of cooling water for the heater. This causes an increase of the running cost of the overall system.
(3) The above-mentioned problems would be encountered. Thus, a flux separating means would be needed. However, in the conventional apparatus, since the generation of vapor and return of the flux are carried out in the same tank, it is necessary to process a large amount of thermal medium. Thus, there is needed a large capacity the flux separating means which increases the cost for the overall system.
Incidentally, such a conventional apparatus is disclosed in, for example, U.S. Pat. No. 4,389,797.